Allow users to control how much system information is disseminated

Description

Currently, the CTCP VERSION requests vend the entire details of the SystemVersion?.plist or ServerVersion?.plist to any IRC user who requests it. For privacy reasons, there may be situations where the user does not want this information known. (For example, the version number could reveal the presence of known security problems.)

This patch adds some hidden defaults to control which pieces are vended:

MVChatHideSystemName - prevents ProductName? from being sentMVChatHideSystemVersion - prevents ProductVersion? from being sentMVChatHideSystemArch - prevents current arch/processor from being sent